The main suspects

ETA The Basque separatist group has denied responsibility for the bombing at Atocha station. In the past ETA - which prides itself on never 'telling lies' - has concentrated mainly on non-civilian targets. Warnings have also always been given in the past and an ETA plot involving rucksacks filled with a similar explosive as Thursday's bombs was thwarted at a train station last December. A plot by a new and more radicalised splinter group cannot be ruled out.

Al-QaedaNow the strongest suspect in the minds of Spain's intelligence services, militants linked to al-Qaeda have the motive and the inclination. Spain is a valid target for Islamist extremists because of its support for the Iraq war and a major attack on European soil has long been expected. The tape of Koranic verses found in the bombers' van backs up this theory as does the lethal timing, co-ordination of the 10 devices, and random nature of the victims. But the attacks were not the usual suicide bombings favoured by al-Qaeda operatives and the claim of responsibility telephoned to an Arabic newspaper made by a known Arab militant group on Thursday has been largely discredited.